152 



BIG MOLECULES 



here; chemical condensations and osmotic pressures, changed as the nuclear 

 membrane disappears and the fluid of the cytoplasm enters, are other 

 candidates. The forces seem to be too long-range to be electrical in nature. 



The essential feature of the replication of the "code" or specification for 

 the animal seems to be the reproduction of the DNA itself. It is now siir- 

 mised that this is a cooperative action of four molecular parts: (a) one of the 

 uncoiled DNA helices, (b) an enzyme, on which has been absorbed (c) the 

 energy carrier, ATP, and (d) the basic polyacid which is to be "stamped" 

 onto (or better: is to condense with) the DNA at the proper spot on the 

 chain. This "enzyme" may be nothing more than one of the proteins syn- 

 thesized already through RNA; it may have a series of "active sites" when 

 uncoiled, one for each of the polyacids which is to be stamped onto the 

 DNA helix. 



Thus, at least in principle, the replication process and protein synthesis 

 have many features in common: 



Replication: DNA + enzyme + ATP -f basic polyacids 



Protein Synthesis: RNA + enzyme + ATP + aminoacids 



The key or code for both is carried by DNA, and thence RNA; and some- 

 times by RNA alone. 



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Figure 6-12. (a) Electron Micrograph of Ribosomes (containing RNA plus small protein 

 molecules called histones) of Escherichia coli: extracted from the pulverized 

 bacteria by ultracentrifugation from a solution 0.01 m in magnesium ions; 

 fixed in formalin; and mounted on carbon-filmed grid negatively stained with 

 phosphotungstic acid to give a dark background. Most particles consist of 

 four segments about 125 A wide. Magnification 1 60,000 x , scale: 0.1 micron. 



